Steeping is cap on.
Airing out or letting it Breathe is cap off.
They do different things to the
juice.
Steeping is allowing entropy to happen which can make flavor more pronounced and/or meld flavorings. (Or it could do nothing)
Breathe is allowing aromatic compounds to escape and oxidize the nicotine. Useful if alcohol needs to be evaporated out, if the more volatile flavoring components need to be removed some, or if you need some of the natural bitter from oxidized nicotine.
Try them both in separate bottles on the same mix and see what happens for yourself. Most likely neither will have much of an impact unless we are talking about
tobacco flavored juices. But they are good tools to have available for the mixer.